Hitherto, there has been developed, to be used for an illuminator or the like a light emitting device which produces white light by making phosphor emit, light by using light emitted from a light emitting element such as an LED element as excitation light.
As such a light emitting device, there is known, for example, a light emitting device which produces white light by using phosphor which emits yellow light through blue light emitted from a light emitting element and mixing the colors of light or a light emitting device which produces white light by using phosphor which emits blue light, green light and red light through ultraviolet light emitted from a light emitting element and mixing the three colors of light.
As a light emitting device having such a configuration, there has been developed a light emitting device in which a light emitting element is directly sealed with curable resin in which phosphor is dispersed. Uses of light emitting devices have expanded to a field which requires high brightness such as a headlight for a car. Because of this and so forth, in recent years, light emitting elements have been high-powered ones and accordingly generate heat. Therefore, when phosphor is provided directly on a light emitting element in such a way as to be dispersed in a sealing agent as described above, the sealing agent may deteriorate by heat generated by the light emitting element.
In order to solve this problem, there is proposed in Patent Document 1 a technique to prevent deterioration of a sealing agent, the technique by which phosphor is dispersed not in resin but in a ceramic solution (ceramic precursor solution) so that a light emitting element is sealed with ceramics which do not deteriorate much by heat.
Although the technique disclosed in Patent Document 1 can provide durability for a light emitting element, when phosphor is mixed with a ceramic precursor solution and the liquid mixture is applied onto a light emitting element, the phosphor precipitates in the liquid mixture because the specific gravity of phosphor is high. Hence, it is difficult to make phosphor evenly exist on a light emitting element, so that released white light is uneven, and accordingly chromaticity varies between light emitting devices.
Further, when an application liquid, in which phosphor is dispersed in a ceramic precursor solution is applied, to a package on which a light emitting element is mounted, a mask component is usually used so that an application region for the application liquid is limited to a predetermined region. However, the phosphor adhering onto a mask component is often discarded. Because phosphor is expensive in general, it is desired that the phosphor adhering onto a mask component is also collected and reused.
The inventors of the present invention gave consideration to this problem and found that the viscosity of an application liquid was able to be increased, phosphor thereof was able to be prevented from precipitating, and evenness of the application was able to be obtained by using polyhydric alcohol having a valence of two or more as a solvent of the application liquid and mixing certain particles therewith. Although, a technique using polyhydric alcohol is introduced in Patent Document 2, the technique disclosed in Patent Document 2 uses polyhydric alcohol as a mere additive (surface hardening retarder) (Paragraphs 0012-0014, 0021, etc.), which is totally different in purpose from the case where polyhydric alcohol is used as a solvent of an application liquid.
However, in the technique by which polyhydric alcohol is used as a solvent of an application liquid in which phosphor is dispersed in a ceramic precursor solution, the phosphor adhering onto a mask component was covered with ceramics formed by the ceramic precursor solution and accordingly fixed onto the mask component, so that the phosphor was not able to be collected or reused.
Then, consideration was given again, and it was decided to apply phosphor and a ceramic precursor solution separately.
More specifically, it was decided (i) to apply onto a light emitting element and dry a liquid mixture in which phosphor is dispersed in polyhydric alcohol to form a phosphor layer and then (ii) to apply onto the phosphor layer and fire a ceramic precursor solution to fix the phosphor of the phosphor layer onto the light emitting element.
By this two-step application, before a ceramic precursor solution was applied, a mask component was collected, and accordingly phosphor was able to be collected from the mask component to be reused.